The present invention relates to an error compensating method of using a three-dimensional measuring apparatus to measure a free-curved surface such as an aspherical lens at a high accuracy of 0.1 to 0.01 .mu.m.
In recent years, a measurement of approximately submicron to 10 nm accuracy is required in measuring the configuration of a free curved object such as an aspherical lens. a conventional contact-type three-dimension measuring apparatus or an interferometer is incapable of measuring an object with such an accuracy. In order to solve this problem, Japanese Laid-Open Patent Publications No. 59-79104 and No. 62-9211 disclose three-dimension measuring apparatuses, respectively capable of measuring an aspherical surface and a free curved surface with a high accuracy. These measuring apparatuses condense light on an object surface to be measured so as to measure the configuration of the object surface using an optical probe.
The three fundamental elements (errors) relating to the degree of accuracy of the three-dimension measuring apparatus are scale, probe, and coordinate axes.
Of these three elements, scale is graduated on three coordinates and can be calibrated according to the length standard determined by the Weight and Measure Act. Therefore, the element of scale is not further described below.
The calibration of the probe error (i.e. an error resulting from an inclination of an object surface to be measured) is made by utilizing one kind of reference sphere in which the roundness and diameter are accurately confirmed, and the calibration of the coordinate axes (i.e. the squareness of each coordinate) is calibrated by utilizing a four-right angle master.
However, the conventional calibration method nevertheless includes a probe error and the error caused by the inaccurate squarenesses of coordinates. These errors are mixed in measured data without being separated from the measured data, which prevents the object from being measured with a high accuracy, and in addition, even though a reference sphere is measured, a perfect calibration of the measurement made by the measuring apparatus cannot be accomplished.
The accuracy of the four-right angle master is limited to approximately one second. Therefore, the four-right angle master is not sufficient to calibrate the squareness of the ultra-high accurate three-dimension measuring apparatus. Also, since the right angle master calibrates the squareness of the three-dimension measuring apparatus in an indirect method, it is difficult for a user to easily calibrate the squareness.
In order to solve the above-described problem, the inventors have measured a reference spherical surface of convex and concave surfaces of the same radius of curvature, and based on the measured data it has been found that an error caused by an inaccurate squareness of coordinates and a probe error depending only on the inclination of an object surface can be separated from each other, and both errors can thus be quantitatively detected respectively.